Modern automotive engines typically utilize a catalytic converter to reduce the exhaust gas emissions produced by the engine. Such converters operate to chemically alter the exhaust gas composition produced by the engine to help meet various environmental regulations governing tailpipe emissions. Catalytic converters typically operate at peak efficiency when the ratio of air and fuel (A/F) entering the converter is within a narrow range centering about stoichiometry.
Electronic fuel control systems are increasingly being used in internal combustion engines to precisely meter the amount of fuel required for varying engine requirements. Such systems control the amount of fuel delivered for combustion in response to multiple system inputs including throttle angle and the exhaust gas composition produced by combustion of air and fuel. Electronic fuel control systems operate primarily to maintain the A/F at or near stoichiometry. Electronic fuel control systems operate in a variety of modes depending on engine conditions such as starting, rapid acceleration, sudden deceleration, and idle. A primary mode of operation is closed-loop A/F control.
Closed-loop A/F control is utilized when certain engine operating conditions are satisfied. Under closed-loop A/F control, the amount of fuel delivered is primarily determined by an estimate of mass air charge. The amount of fuel is then modified by a value related to the concentration of oxygen in the exhaust gas, such concentration being indicative of the fuel-air composition that has been ignited. The resulting quantity of fuel injected into the engine corresponds precisely to the engine operating conditions and results in lower tailpipe emissions.
In closed-loop A/F operation, the oxygen in the exhaust gas is sensed by an oxygen sensor. The electronic fuel control system adjusts the amount of fuel being delivered in response to the output of the oxygen sensor. A sensor output indicating a rich air/fuel mixture (an A/F below stoichiometry) will result in a decrease in the amount of fuel being delivered. A sensor output indicating a lean air/fuel mixture (an A/F above stoichiometry) will result in an increase in the amount of fuel being delivered.
In conventional closed-loop electronic fuel control systems employing switching-type oxygen sensors, the A/F will oscillate above and below stoichiometry at a limit-cycle frequency determined by the characteristics of the system. Such operation will generally keep the catalytic converter operating at its peak efficiency, thereby reducing tailpipe emissions. However, if an A/F transient error is imposed on known fuel control systems, the exhaust A/F will shift away from stoichiometry for a certain time period until the feedback signal can correct the error. During the time that the A/F is shifted away from stoichiometry, the efficiency of the catalytic converter will be reduced and its ability to chemically alter the exhaust gas produced by the engine will be diminished. As a result, tailpipe emissions will increase until the catalytic converter subsequently regains its full capacity with oscillation of lean and rich exhaust gas composition around stoichiometry.